Embodiments of the present invention relate to thin film batteries and their methods of manufacture.
A thin film battery 20 typically comprises a substrate 22 having one or more thin films 24, 26, 28 thereon, as for example, shown in FIG. 1. In a conventional thin film battery 10, typically, a cathode current collector 24 is deposited on the substrate 22, and thereafter, a cathode 26 is deposited on the cathode current collector 24. An electrolyte 28 is formed in contact with the cathode 26, and an anode (not shown) and optional anode current collector (also not shown) are on the other side of the electrolyte 28. The thin films are typically formed by thin film fabrication processes, such as for example, physical or chemical vapor deposition methods (PVD or CVD), oxidation, nitridation or electro-plating, on a substrate that is has good mechanical strength. The thin film battery is typically formed by thin film processes such as physical or chemical vapor deposition methods (PVD or CVD), oxidation, nitridation, plating, or other such processes.
It is desirable for the cathode 26 to have a crystalline microstructure. When the cathode 26 comprises a thin film having an amorphous or microcrystalline structure, the energy that can be stored in such films is usually less than that stored in a microcrystalline film. Furthermore, the charge and discharge rate of the amorphous or microcrystalline film is also smaller than that of a crystalline material film with the same chemical composition. To crystallize an amorphous or microcrystalline thin film to form the cathode 26, the as-deposited thin film is annealed in a separate process step. The crystallization or annealing temperature that is required to crystallize the amorphous oxide film may be a relatively high temperature. For example, the crystalline microstructure of a thin film cathode comprising LiCoO2 is dependent upon an annealing step that is conducted subsequent to deposition of an amorphous or microcrystalline thin film of LiCoO2. The typical annealing temperature is about 700° C. The high temperature annealed crystalline LiCoO2 provides good cathode performance, such as high energy density (0.07 mAh/cm2/mm) and high charge to discharge current (more than 5 mA/cm2).
Low temperature processes that produce high quality crystalline LiCoO2 cathode materials have also been developed, for example, to deposit LiCoO2 in at least a partially crystalline form. A 200 to 600° C. low temperature anneal process step in oxygen improves the performance such the as-deposited LiCoO2 to that of a high temperature annealed cathode material.
However, in both the high and low temperature processes for making the cathode 26, oxidation of underlying cathode current collector 24 is a problem. The annealing process, which is often carried out in a flow of oxygen, limits the materials that may be used to form the underlying current collector 24 because of melting, oxidation, or inter-diffusion problems. This problem may be reduced by making the cathode current collector 24 out of a noble metal, such as Pt or Au. However, such metals increase the cost of battery 20. Also, the annealing process can generate thermal stresses due to the thermal expansion coefficient difference between the substrate 22, cathode 26, and cathode current collector 24. These stresses can result in peeling or de-lamination of these layers from the battery 20.
Thus it is desirable to have a battery having a cathode and cathode current collector capable of providing good properties, such as for example, desirable energy storage and conductor properties, respectively. It is further desirable to be able to reduce the cost of fabrication of the battery. It is also desirable to be able to minimize any thermal stresses which may be caused by annealing of thermally mismatched materials in the fabrication of the batteries.